Sabellid Polychaete Infestation Disease in Abalone
Category 1 (Not Reported in Canada)
Common, generally accepted names of the organism or disease agent
Scientific name or taxonomic affiliation
Terebrasabella heterouncinata (named by Fitzhugh and Rouse 1999) is a member of the polychaete subfamily Fabracidae.
Believed to occur in most abalone culture facilities (including an offshore production facility) in California, USA and Baja California, Mexico. Although this polychaete colonized test molluscs held adjacent to an effluent outlet from an affected abalone hatchery, the polychaete has not yet been observed in wild abalone stocks in California. It has been speculated that this sabellid is not native to California and was probably introduced with cultured abalone from South Africa that were not quarantined on arrival in California in the late 1980s. This sabellid also caused problems in South African abalone farms among Haliotis midae, especially when high abalone stocking densities were combined with poor hygiene and marginal water quality in culture tanks (Cook 1998; Ruck and Cook 1998, 1999; Sales and Britz 2000; Simon et al. 2004). Recently, Terebrasabella heterouncinata was reported from cultured Haliotis rufescens in Iceland (2002, 2005 and 2006 Health Certificates from Dr. Gisli Jonsson, Keldur, Reykjavik, Iceland) and was detected infesting this species of abalone in an abalone aquaculture facility in Puerto Montt, southern Chile (Moreno et al. 2006).
Haliotis rufescens, Haliotis fulgens, Haliotis corrugata, Haliotis midae and other species of abalone as well as other species of marine gastropods including snails and limpets (Kuris and Culver 1999). Bivalves rarely, if ever, become infested. Sabellid larvae will not occupy empty abalone shells but infestations acquired prior to abalone death can persist, remain viable and reproduce in the remaining shell.
Impact on the host
Although not directly life threatening to abalone, this sabellid polychaete has become a serious pest of abalone in aquaculture facilities in California. Detrimental effects include significantly reduced growth rates, grossly deformed shells with the absence of respiratory aperture (gill pore) formation on the leading edge of the shell, decreased meat yields, increased mortality due to the inability of the abalone to right themselves when dislodged from their substrate, and reduced marketability. The juvenile sabellid attaches itself to the shell at the growing edge and covers itself with a mucus tube and a thin membrane. As the abalone secretes new shell it covers the juvenile polychaete with aragonitic nacre but the polychaete keeps the area around it's anterior end open to the environment. The aragonitic nacre is deposited over the sabellid tubes by the abalone within 12 hours, about 4 to 8 times faster than normal aragonitic synthesis. When the abalone attempts to form the prismatic calcite layer of the shell in the vicinity of the sabellid, the polychaete interferes with the process. The extent of the suppression of prismatic calcite deposition may depend on the number of juvenile sabellids that became established (Day et al. 2000). Heavy infestations (as observed in some abalone facilities) result in shells with a thickened leading edge that is very fragile and porous, due to the lack of prismatic calcite and the honeycombed affect caused by high polychaete populations. The shell of heavily infested abalone grows downward instead of outward as in normal abalone.
Usually the hermaphroditic adult broods six to ten orange eggs at the posterior end of the tube. After hatching, the larvae crawl out the opening of the tube. Larvae spread from one abalone to another when the abalone come in contact with each other but, there is evidence to suggests that the crawling benthic larvae may also be disbursed short distances via the water column, on vegetation or along the substrate (e.g. larvae were transmitted up to 180 cm over a sandy substrate (LeBlanc 1995)). Water flow rates of greater than 10 liters per minute seems to reduce larval dispersal upstream. In experimental trials, larger abalone (15 mm shell length) became infested more often than smaller abalone (6 mm shell length) (LeBlanc 1995) but infestations have occurred in abalone about 2 mm in shell length. Experimental results seem consistent with observations in abalone culture facilities, because the prevalence and intensity of infestation is usually higher in the larger abalone. However, slower growing abalone appear to be more susceptible to heavy infections. A fast growing abalone can encapsulate a small number of sabellids and extend it shell beyond them. In an abalone culture facility, infestations were reported to rapidly build up to a damaging level within 60 days, indicating a brief generation time for this polychaete. However, the results of life history laboratory experiments by Finley et al. (2000a and b, 2001) indicated that 50% of the sabellid larvae exposed to H. rufescens for 24 hours, began to feed by day 6, 5 and 4, became sexually mature by day 83, 68 and 49 and produced larvae by day 298, 165, and 111 at temperatures of 11, 16 and 21 °C respectively. Also, the products of at least two generations of self fertilization are fully functional organisms. Concerns about the spread of this coloniser have interfered with the reseeding plans for rehabilitation of over-utilised abalone populations in California (Hauser 1997).
Gross Observations: Infested abalone have a characteristic dome shaped shell with damage and deformation as described above. The branchial crown (about 1 mm in diameter) of the sabellid may be observed along the leading edge of the shell when the abalone is submerged in sea water. The orange ovoid eggs (about 0.25 mm long) can sometimes be seen at the end of the tubes from the interior side of the abalone shell. Infestations can be detected by using one of the four following techniques:
1) Examine the ventral, inner margin of the abalone shell for larvae and/or tubes indicative of new infestations. Tubules appear as white thick marks perpendicular to the shell margin. These marks can be confused with natural growth striations in the shell. Infection can be confirmed by examining the leading edge of the tracks for an aperture and the presence of polychaets with a dissecting microscope. Also, a physical rise in the shell indicates a worm tube, whereas natural striations are grooved or indented. Four weeks after infestation, the tubes may not be easily recognized by mere examination of the inner surface of the shell. Thus, this method is only adequate to examine for new infestations. Previous infections can be detected by shucking the abalone and examining the entire inner surface of the shell for old sabellid marks that may appear as dark tracks in the area of the muscle scar because the nacre on the inner surface of the muscle scar becomes thin as the abalone grows.
2) Examine the outer surface of the shell especially along the ventral edge for sabellid feeding crowns and/or sabellid tube openings (roughly circular in shape and uniform in size and may have part of the mucous sheath extending through the opening). The sabellid crowns can only be viewed when the abalone is submerged in a bowl of seawater and the shell examined with a dissecting microscope. Tentacular extension can be stimulated by adding a small amount suspended material (e.g., planktonic algae or homogenized abalone food) to the bowl.
3) Inspect pieces of shell, broken with a hammer in a bowl of sea water, for tubes and/or eggs, larvae and adult sabellids. This method is primarily useful for moderate to high infestations because it is difficult to find small numbers of sabellids by crushing the shell into small fragments. However, this technique is useful for determining the reproductive status of the sabellids in an infestation and thus the potential for new infestations can be assessed.
4) Inspect acid-dissolved shell for sabellids by removing the calcium carbonate of the shell without damage to the proteinaceous matrix and associated polychaetes. Submerge shell in a gently stirred solution of formalin, nitric acid and freshwater for five minutes or more depending on the size of the shell. When the shell is decalcified, peel back the protein matrix to reveal the sabellids which will have been fixed (preserved) by the formalin.
Wet Mounts: Adult sabellid polychaetes are about 4 to 5 mm in length with a branchial crown, 8 thoracic segments and 3 abdominal segments. The first thoracic segment has an anterior dorsal collar that is bifurcated mid-dorsally by the faecal groove. The actively crawling benthic larvae, about 1 mm in length, lack a branchial crown.
Methods of control
The ability of the sabellid to withdraw into it's tube makes it very difficult to eliminate without harming the abalone. Even if the adult hermaphrodite polychaete could be destroyed, there are often many fertilized eggs left in the tubes to hatch later. Failed methods for controlling infestations include: air exposure for extended periods of time, freshwater treatment, severe temperature exposure, chlorine treatment, and exposure to several insecticides. Lower water temperatures result in a decrease in polychaete reproduction but will also reduce abalone growth. Leighton (1998) reported that high temperatures (48 h uninterrupted exposure to 28.5 °C) killed all stages of the sabellids with only minor mortalities among abalone species tolerant to these temperatures (e.g., H. fulgens and H. corrugata). However, other workers have observed that a small percentage of the sabellids can survive this high temperature treatment (C. Friedman, California Dept. Fish and Game, Bodega Marine Laboratory, P.O. Box 247, Bodega Bay, CA 94923, USA, personal communication). According to D.L. Leighton (Marine Bioculture, Leucadia, CA and Carlsbad Aquafarm, P.O. Box 2600, Carlsbad, CA, 92018, USA, personal communication), an incident of "reinfestation" was attributed to the unintentional mixing of thermal treated abalone with a few untreated individuals and that rigorous application of the thermal treatment process has resulted in the elimination of sabellid infestation from a previously contaminated abalone culture facility. Nevertheless, this thermal treatment will not be suitable for abalone species that have upper lethal limits well below 28 °C (e.g., H. rufescens and H. kamtschatkana).
Oakes et al. (1995) proposed dipping the shells of affected abalone in molten paraffin wax to cause anoxia and death to sabellids within the plugged burrows. However, the proper application of molten wax to abalone shells in a large production facility is problematic (labour intensive) and treated abalone can become reinfested because waxing does not remove the sabellids from the facility. Shields et al. (1998) proposed that the application of lipid-walled microcapsules (LWMs) had potential for controlling sabellid infestations in abalone culture facilities. The use of LWMs takes advantage of the filter-feeding nature of the polychaetes while avoiding impact on the abalone which are benthic herbivores. Although the majority of the polychaetes readily consumed and digested the LWMs, the appropriate toxin(s) for incorporation into the LWMs to control polychaete populations with minimal impact on the abalone and the environment has yet to be identified. Loubser and Dormehl (2000) proposed the use of ultrasound in the treatment of sabellid infestations in South African abalone. Preliminary result were promising but, application of the treatment on a commercial scale remains to be tested.
Because only heavy infestations result in shell deformation, the impact of the disease on a culture facility can be reduced by aggressively culling juvenile abalone that are slow growing and removing abalone with signs of sabellid infestation. In addition, the problem is being addressed by inspection of seed before sale, improved farm sanitation practices and use of screens on farm discharges. Tanks can be disinfected with freshwater baths for 24 hours (larval sabellids were unable to infest susceptible gastropods at a saltinity of 17 ppt and larvae were unable to crawl following a 10 second exposure to freshwater) and/or thorough drying for several weeks. Culling of infested stocks and strict hygiene protocols including freshwater treatment of tanks has proved effective in curbing new infestations in California (Finley et al. 2003). Facilities free of this pest should not obtain abalone for stocking from facilities where this pest has been observed.
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Bower, S.M. (2006): Synopsis of Infectious Diseases and Parasites of Commercially Exploited Shellfish: Sabellid Polychaete Infestation Disease in Abalone.
Date last revised: December 2006
Comments to Susan Bower
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